This invention relates to a valve assembly for a hydraulic system. In particular, the invention relates to a valve assembly for a hydraulic system of the type having a fluid flow circuit including a fluid supply source, a motor, and a pump for circulating fluid in a forward or reverse direction to thereby operate the motor in a forward or reverse direction.
Various hydraulic systems incorporate valving and circuitry to control the forward-reverse operation of hydraulic motors. When such motors are used for example, as the elevator drive of self-loading earthmoving scrapers it is desirable to isolate motor operation from external forces that tend to rotate the motor. In order to obtain optimum operation, it is also desirable to provide a make-up valve in the system to prevent cavitation of the motor due to overrunning, such as frequently occurs with the valves in neutral position and external loading of the motor.
Frequently, prior art attempts to provide a workable forward and reverse fluid flow to the motor in such hydraulic systems have failed to include means for isolating the motor from external forces and means for preventing cavitation of the motor. Further, many such prior art attempts have resullted in cumbersome hydraulic circuitry and additional valve components, all of which increase the overall cost of the control system.
Exemplary of such prior art systems are those described in U.S. Pat. Nos. 3,164,171; 3,198,099 and 3,411,521.